The invention relates to an image projection system comprising an illumination system for supplying an illumination beam, a modulation system for modulating said illumination beam in conformity with image information to be projected, and an optical system for projecting an image, the modulation system comprising at least one liquid crystalline image display panel having a first and a second polarizer between which a layer of TN (twisted nematic) liquid crystalline material is enclosed.
The invention also relates to a head-mounted display.
An increasingly higher image quality is aimed at in future image projection systems with liquid crystalline image display panels. The image contrast on the projection screen is also determined by the absorption which can be realized in the dark state of the panels. Pixels of a TN-LCD (twisted nematic liquid crystal display) are maximally a transparent if there is no voltage across the pixels. In an image, these pixels are in a bright state. Pixels across which a voltage is applied absorb light and are thus in the dark state. However, the absorption is limited and is not the same for all directions of incidence of the light beam. Consequently, the dark state may still be relatively bright, notably for directions of incidence of the light for which the absorption is worse. This gives rise to a relatively poor image contrast. Moreover, a poor contrast of the image display panel has detrimental effects on the color reproduction.
The image contrast is determined, inter alia, by the transmission characteristic of the image display panel for the directions from which the light beams are incident and consequently also by the illumination profile which is incident on the image display panel. Moreover, only the light which is incident on the projection lens within a given angle will be passed. This is referred to as the acceptance angle. A maximal display panel contrast is aimed at within this acceptance angle.
The viewing angle dependence of the image contrast as a consequence of the dependence of the image display panel on the direction of the illumination beam results from the residual retardation of the liquid crystalline material. Light which propagates in directions which are not parallel to the director of the liquid crystalline material will undergo a small birefringence. An image projection system of the type described in the opening paragraph is known from, for example, European patent application EP 0 390 511, in which this problem is solved by providing at least one additional optically transparent element of a positive birefringent material at the liquid crystalline layer, with optical properties which compensate for said residual retardation in a single viewing direction.
The known system has the drawback that the contrast is only optimized within a very narrow cone of viewing directions, with the result that the high contrast area is only shifted in position. Moreover, the parameters of the compensation elements have a very small tolerance.
It is an object of the present invention to provide an image projection system in which a relatively high image contrast can be realized, obviating the above-mentioned drawbacks.
To this end, the image projection system according to the invention is characterized in that the image display panel comprises a single birefringence-compensating element between the layer and one of the two polarizers, which element has a tilted optical director profile whose projection in the plane of the polarizers encloses an angle xcfx86 different from 0 with the active rubbing direction of the layer.
The active rubbing direction is understood to mean the rubbing direction which determines the contrast in the viewing directions perpendicular thereto, in which directions the contrast is to be improved. The contrast in a given viewing direction is determined by the part of the liquid crystalline layer whose rubbing direction is substantially perpendicular to this viewing direction.
The present invention is based on the recognition that the fact that a relatively small viewing angle with respect to the illumination system is sufficient for a liquid crystalline image display panel in an image projection system can be used to great advantage. The high contrast area is to be widened and shifted towards the directions from which the main illumination beams illuminating the image display panel originate. The viewing angle within which the contrast is to be improved is determined by the angular spread of the illumination beams and by the acceptance angle of the projection lens.
Since, for projection, a high contrast area is sufficient which extends in the direction of the main illumination beams of the illumination system, which is in contrast to direct-vision systems in which the high contrast area is to extend both in the horizontal and the vertical direction, it is sufficient to use a single birefringence-compensating element. Moreover, the element is, rotated so that the optical axis of the element encloses an angle xcfx86 different from 0 with the active rubbing direction of the liquid crystalline material. The drastic change of the symmetry of the contrast curves thus does not have any detrimental effects for projection applications. Since xcfx86 is different from 0, an improved contrast is now also possible upon perpendicular incidence.
Since use is made of a single element, the material costs are reduced and less absorption will occur in the bright state. Moreover, the tolerances are less strict than in the known systems.
A preferred embodiment of the image projection system according to the invention is characterized in that the birefringence-compensating element is an element having a negative birefringence.
Since liquid crystalline material has a positive birefringence, compensation with an element having a negative birefringence and a tilted optical director profile is simplest. Moreover, this element will ensure that the high contrast area is enlarged in the desired viewing directions.
It is to be noted that it is known per se from, for example, U.S. Pat. No. 5,583,677 to provide an image display panel with a negative birefringent foil. Here, however, in the case of a single element, this element is positioned at an angle of xcfx86=45xc2x0 so that there is only a general widening which is not optimized for a maximum contrast in the directions from which the main light beams illuminate the image display panel. Moreover, the tolerances are very strict because the optical axis of the foil is positioned at an angle of 45xc2x0 with respect to the polarizer.
A further embodiment of the image display device according to the invention is characterized in that 0  less than |xcfx86|xe2x89xa615xc2x0.
Since, in the present invention, |xcfx86| is only a few degrees, the contrast of the image display panel is considerably less sensitive to variations of thickness, tilt and retardation.
A further embodiment of the image projection system according to the invention is characterized in that the element is present on that side of the liquid crystalline material where said material has its active rubbing direction.
At this position, the element will have the optimal effect.
A negative birefringent element with a tilted optical director profile may be obtained in different manners.
A first embodiment of the image projection system according to the invention is characterized in that the element comprises a negative birefringent foil having a tilted optical director profile.
A second embodiment of the image projection system according to the invention is characterized in that the element comprises a plurality of positive birefringent foils, at least one of which has a tilted optical director profile.
The image projection system described above may be, for example, an LCD projection system or a head-mounted display. In the case of an LCD projection system, the optical system consists of a projection lens system. In both applications, it may be very advantageous to widen the high contrast area and shift it towards given viewing directions without the attendant change in the contrast curve having detrimental consequences.
These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.